1. Field
The present invention pertains generally to apparatus for monitoring the power distribution within the core of a nuclear reactor, and more particularly, to an ion chamber detector having an enhanced fission gamma radiation response.
2. Related Art
In many of the aged reactors presently in operation reliance for nuclear flux measurements, as used in the reactor control and protection systems, has been placed on out-of-core detectors supplemented by an in-core flux mapping system, which has been in use since as early as 1967; with the latter originally intended to provide proof of the core design and some calibration measurements.
With a growing trend toward larger reactor cores, there has been some concern as to the possible adverse affects of core power distributions and whether out-of-core detectors could adequately detect such possible adverse power distributions. Test data showed that the split section out-of-core detectors, presently in use, responded to axial tilt, but the accuracy of the tilt measurements were affected by the geometry and construction materials employed at the detector wells and by the spacing between the vessel and the detectors. Correction factors have been developed for these affects, but there was some question as to whether out-of-core detectors will in all cases provide an adequate alarm of an adverse power distribution.
To obviate the foregoing concern, a method was developed for automatically monitoring the power distribution employing the moveable in-core detectors by providing a more accurate, detailed, automatic, frequently updated, data readout of the reactor core power distribution. The method, taught in U.S. Pat. No. 3,932,211, issued Jan. 13, 1976 and assigned to the Assignee of this invention, inserts the moveable detectors into the reactor core region during normal power operation according to a predetermined, intermittent, timed program. The measurement system that performs the periodic core power distribution measurements typically controls the simultaneous insertion and withdrawal of as many as six moveable detectors until measurements are obtained from all of the prescribed core radial locations. Each detector used is inserted through a common radial location to ensure that the detector sensitivities can be normalized to allow the production of an accurate “relative” core power distribution from detectors having different absolute sensitivities.
Preferably, a plurality of moveable detectors are arranged in electrically redundant groupings and are normally stored within the reactor thermal environment outside of the core reactivity region to minimize thermal cycling. In operation, the detectors are driven into the reactor, through the reactor vessels' lower head, through the core support plate and through prescribed fuel assembly bottom nozzles to the fuel assembly instrumentation tubes through which the detector is extended to the desired core elevation. As dictated by the predetermined, time program, alternate groupings of detectors are driven along corresponding linear paths within the instrumentation thimbles within the core at staggered time intervals governed by the reactor core physics. The programmed detector drive sequence is automatically reinitiated upon a given controlled reactivity change to provide the most meaningful data input to the reactor operator.
Moveable in-core detectors are now used by both boiling water reactors and pressurized water reactors to perform periodic detailed measurements of the core power distribution. The moveable detectors used are either primarily sensitive to neutron or gamma radiation. The type of detector most commonly used in both pressurized water reactors and boiling water reactors is a fission chamber style of detector. In this design, the signal output from the detector is directly proportional to the thermal neutron population surrounding the detector. The thermal neutron population is directly proportional to the local fission rate and local core power level. This response is generated by the use of significant amounts of highly enriched U235 in the construction of the detector. Since U235 is a special nuclear material, the cost to purchase and operate the moveable fission chambers is quite high. The moveable fission chambers are also quite delicate, so they are subject to frequent mechanical failure. The major technical benefit associated with their use is the direct relationship between the output signal and the local thermal neutron population and the direct relationship that the thermal neutron population has with the local core power production rate. FIG. 1 provides a layout schematic of a miniature fission chamber 10. The miniature fission chamber has a stainless steel tubular casing 12 that is capped at both ends and forms an outer electrode. Al2O3 ceramic insulators 16 support a central mineral filled coaxial output electrode 18, which is insulated from the outer electrode 12. The stainless steel casing 12 surrounds a central chamber 14 that is filled with an Argon filler gas 22 with the walls of the chamber 14 coated with 90% enriched U235 and U3O3. A detector bias voltage 32 of between 20 and 150 volts DC is maintained between the two electrodes. In operation, an incident thermal neutron 28 causes a fission event 30 within the enriched U235 resulting in high energy ionizing fission fragments 26 which create ionized gas molecules 24 within the Argon gas. The voltage bias on the central electrode 18 collects the ionized gas particles 24 resulting in a detector output 34 which is proportional to the fission events 30 resulting from the incident thermal neutrons 28.
A moveable detector design using a miniature ion chamber 10 has been recently introduced for use in a boiling water reactor. This type of detector also produces a signal proportional to the local core power, but the signal is stimulated by gamma radiation interactions in the ionization chamber region of the detector. This type of detector does not require U235 as the stimulation for the output signal, so the cost and upkeep of this type of detector is significantly less than for a fission chamber style. The detectors also tend to be more rugged than fission chamber moveable detectors. The most limiting issue associated with the use of an ion chamber detector is the much lower signal output corresponding to a given local core power level. The use of this style of detector requires the use of very sensitive and expensive signal processing electronics. There is also an uncertainty that needs to be included in the core power distribution measurement uncertainty to account for the non-single valued relationship between output signal and local fission rate or core power. This power distribution measurement uncertainty increase potential may actually reduce the maximum power output that can be achieved by the reactor operator.
FIG. 2 provides a longitudinal cross section of a layout schematic of a miniature ion chamber 36 with FIG. 2B showing a cross section taken orthogonal to the sectional view shown in FIG. 2A. The miniature ion chamber 36 has an outer metal casing 38 that forms the outer electrode and insulated end caps 40 and 42 that support a central electrode 44. Similar in construction to the miniature fission chamber 10, the casing 38 surrounds a central chamber 50 that is filled with a fill gas 48.
Each style of moveable detector has suboptimal performance characteristics that significantly increase the cost of operation of the detector system. Accordingly, an improved detector is desired that is more rugged and less expensive to operate than those currently employed.
Additionally, such a detector design is desired that will minimize the uncertainty that has to be factored in to the core power measurements.